Mutations in mitochondrial DNA lead to a spectrum of neurodegenerative diseases for which no treatment exists. Almost all of them involve ocular structures that include the optic nerve, retina, extraocular muscles or eyelids. The most common of these is Leber Hereditary Optic Neuropathy (LHON) caused (in half the cases) by a mutated ND4 subunit gene of complex I in the respiratory chain. Our goal is to develop gene therapy for this mitochondrial disease by delivery of genes encoding the normal ND4 subunit to affected cells and tissues. We propose to design, modify and test an adeno-associated virus (AAV) to which a mitochondrial targeting sequence is appended to the viral envelope for delivery of its payload of DNA (a normal ND4 subunit gene), directly to the mitochondrion for rescue of cultured LHON cells and then of our animal model of LHON. Our mouse model shows optic nerve head swelling followed by optic atrophy and degeneration of retinal ganglion cells, which are characteristics of LHON patients. We developed this LHON- model by modifying the mutant human ND4 gene for expression in the nucleus then directed it to the mitochondria with a targeting sequence. Our Specific Aims are: 1) To (a) direct the AAV virion to mitochondria of cultured cells and test for the presence and expression of the payload DNA and then (b) test this strategy for the rescue of defective respiration of LHON cells. 2) To test this strategy for delivery of the human ND4 gene to the LHON-model eye for rescue of retinal ganglion cell degeneration and optic neuropathy. As proof of the feasibility of our project we provide extensive preliminary data showing that our first generation vector is targeted to mitochondria, is expressed in cultured cells as well as the murine visual system, and that it rescues cultured LHON cells. The novel technology developed here may deliver virtually any mitochondrial gene, and thus may provide the platform to treat not only visual loss, ophthalmoparesis and ptosis, but also the myriad of disabilities including premature death experienced by patients with diseases caused by mutated mitochondrial DNA. We will share the vector with other groups whose goal it is to treat these disorders.